In the time-varying channel of land mobile telecommunication, bit errors often occur in burst. This is due to the fact that a burst of successive bits may be corrupted by a long lasting deep fade. However, only random errors or a short error burst can be effectively corrected by common forward error control (FEC) coding techniques. To solve this problem, a method that spreads FEC coded bits of a message over a long packet is desired, that is, successive (coded) bits of a message are sent in a non-successive way. In this way, even though a burst of bit errors occurs in the communications process, they shall be de-spread at the receiver to a more random error pattern such that the random errors are readily corrected by error correction function, and the original message is recovered. This method is interleaving technology.
ITU G.9902, ITU G.9903, IEEE P1901.2, and G3-PLC are Power Line Communication (PLC) international standards based on Orthogonal Frequency Division Multiplexing (referred to as “OFDM”), wherein a design of Forward Error Control (referred to as “FEC”) interleaver that is used can provide protection against following two different sources of errors:
Several consecutive OFDM symbol errors (time domain) caused by strong impulsive interference;
Several consecutive OFDM subcarrier errors (frequency domain) caused by strong frequency selective fading or narrow-band interference.
In order to improve robustness of the PLC communications, repetition code is often used in the above PLC standards. Interleaving is done after the repetition and copying process, intended to fight both above problems at the same time, providing diversity gain in both the time domain and the frequency domain. Given the number of effective subcarriers (m), the bits to be interleaved are arranged into an n-by-m matrix by a channel interleaver, where n is the number of the OFDM symbols. Interleaving is done in two steps. In the first step, each column of the interleaving matrix is circularly shifted a different number of locations to prevent a whole column data corrupted by strong frequency selective fade or narrow-band interference. In the second step, each row of the interleaving matrix is circularly shifted a different number of locations. Therefore, corrupted OFDM symbols are spread over different symbols. The total number of circular shifts is determined by the parameters mi, mj, ni and nj, which are selected based on the number of subcarriers in each OFDM symbol (m) as well as the number of the OFDM symbols (n).
FIG. 1 shows bits arrangement of original permutation matrix in a buffer of the interleaver using above standards. Wherein, original bit position in the original permutation matrix is (i, j), where, i=0, 1, . . . , m−1, j=0, 1, . . . , n−1. Interleaved bit position in the permutation matrix is (I, J), and the relationship between the two bit positions is as the following formula:I=(i·mi+J·mj)mod m J=(j·nj+i·ni)mod n where (mi, mj) and (ni, nj) are selected as the following formula:GCD(mi,m)=GCD(mj,m)=GCD(ni,n)=GCD(nj,n)=1where GCD(a, b) indicates the greatest common divisor of two positive integers, a and b. A good set of above parameters can be found based on the following two parameters m and n by performing a simple search and communication protocol or standard will set the rule of generating these parameters. Wherein, m is the number of subcarriers comprised in each OFDM symbol, n is the number of OFDM symbols comprised in the interleaving data block.
Both ITU G.9902 and G3-PLC standards propose a look-up table (LUT) method to implement the double-circular permutation interleaving procedure. However, for G3-PLC interleaver, the range of allowable frame lengths (K=n*m) can be quite large, and furthermore, the LUT size depends on n and m which are unknown to the receiver, therefore the interleaving table must be generated on-the-fly after an FCH (frame control header, referred to as “FCH”) has been received. Therefore, the straightforward LUT approach not only requires a large amount of memory space for interleaving tables but also wastes unnecessary computational complexities.
Research background relating to the present invention can specifically refer to the following materials:
1. G3-PLC Physical Layer Specification, ERDF (Electricite Reseau Distribution France), August 2009;
2. Low Frequency (less than 500 kHz) Narrowband Power Line Communications for Smart Grid Applications, IEEE P1901.2 standard, August 2013;
3. Narrowband orthogonal frequency division multiplexing power line communication transceivers for ITU-T G.hnem networks, ITU G.9902 standard, October 2012;
4. Narrowband orthogonal frequency division multiplexing power line communication transceivers for G3-PLC networks, ITU G.9903 standard, October 2012.